Recent requirements of the electronics industry for purer gases have increased interest in the use of zeolitic adsorbents for removing trace nitrogen from argon. Polyvalent chabazites have been disclosed as useful adsorbents for removing trace nitrogen from argon, for purifying methane, and for quantitatively separating oxygen from argon in chromatographic applications. The intrinsic properties of calcium-exchanged chabazites, realized only after proper dehydration, expand the range of contaminant gases which can be removed economically from bulk gases using standard adsorption techniques.
However, the availability of high-grade chabazite is extremely limited. Pure chabazite exists only rarely in nature and is too expensive to be used as a commercial adsorbent for large scale processes. In the invention herein, chabazite is meant to include a large number of synthetic zeolites having the chabazite topology and included under the general IUPAC structure code of chabazite (CHA). Synthetic analogues of chabazite are known. Examples include zeolites D, R, G, and ZK-14 (Breck, Zeolite Molecular Sieves, John Wiley and Sons, New York, N. Y., p 110 (1974) and Cartleidge, et al., Zeolites, 4, 226 (1984)). These phases could have minor variations in their crystal structure. Known methods for preparing synthetic chabazites having the preferred composition are not useful commercially since they suffer from low yields, poor product purity, long cystallization times, and difficult, if not impossible, scale-up. Before chabazite-based adsorbents can be exploited commercially, a synthetic method for preparing a pure chabazite by an economically attractive process is needed.
Some workers in the field have proposed methods for preparing synthetic chabazites. For example, Barrer, et al. describe the preparation of Zeolite G (J. Chem. Soc., 2882-2891 (1956) and J. S. C. Dalton, 1254-1259 (1972). The potassium form of these zeolites can be prepared from a variety of silica alumina sources which produce a number of phases of different composition (SiO.sub.2 /Al.sub.2 O.sub.3 1.0 to 4.5). However, such preparations only worked in very dilute gels and required up to several weeks to crystallize.
Zeolite R, a synthetic chabazite-like phase first prepared by Milton (British Patent No. 841,812 (1960)), only forms in the limited composition range of SiO2/Al203=3.44 to 3.66. However, we have found that synthesis of zeolite R using methods taught by Milton is a kinetically controlled process which does not lend itself to scale-up. Because temperature, crystallization time, agitation, reagent source, and scale are important and even interdependent, it has not been possible to identify conditions under which such a method could be used to produce commercial quantities of pure synthetic chabazite reproducibly. Other methods of synthesis suffer similar problems on scale-up. For example, the method reported by Tsitsishvili, et al. (Soobshcheniya akademii nauk Gurzinoskoi SSR, 97, No 3, 621-624 (1980) produces chabazite contaminated with erionite and/or zeolite EAB when practiced on a large scale.
Another method for preparing synthetic chabazite designated ZK-14 is described by Cartleidge, et al. in Zeolites, 4, 218 (1984). This method is carried out in very dilute gels using high levels of tetramethylammonium hydroxide as a template, and produces low yields of zeolite per unit volume of reactor at high reagent cost.
While the tetramethylammonium cation (TMA) as discussed by Lok, et al. in The Role of Organic Molecules in Molecular Sieve Synthesis, Zeolites, 3, 282-291 (1983) is known to have structure-directing and gel chemistry-altering effects, it is nevertheless known to be poisonous as well as too costly for most commercial applications (See the discussion by Narita, Ind. Eng. Chem. Prod. Res. Dev., 24, 507-512 (1985)). It is, therefore, desirable to reduce the amount of TMA required to form chabazite.
There is no known method for producing commercially significant quantities of chabazite. Accordingly, a method for the rapid preparation of a nitrogen-adsorbing synthetic chabazite of the desired structure on a large scale in high yield from readily available starting materials is needed.